This invention relates generally to refrigeration systems and more particularly concerns placement of an accumulator in a multievaporator refrigeration cycle to increase the efficiency thereof.
Conventional refrigeration systems used in household refrigerators operate on the simple vapor compression cycle. Such a cycle includes a compressor, a condenser, an expansion throttle, and an evaporator connected in series and charged with a refrigerant. A conventional household refrigerator, of course, has two food compartments, the freezer and the fresh food compartment. The freezer is generally maintained between -10.degree. F. and +15.degree. F., the fresh food compartment is preferably maintained between about +33.degree. F. and +47.degree. F. To meet these requirements, the evaporator of the typical system is operated at approximately -10.degree. F. The refrigeration effect is captured by blowing air across the evaporator. This air flow is controlled so that a portion of the air flow is directed into the freezer and the remainder is directed into the fresh food compartment. Thus, the refrigeration cycle produces its refrigeration effect at a temperature which is appropriate for the freezer but lower than necessary for the fresh food compartment. Since more mechanical energy is required for cooling at lower temperatures, the refrigeration system described above uses more mechanical energy than one that produces cooling at two temperature levels. However, the well known procedure of employing two independent refrigeration cycles, one to serve the freezer at a low temperature and another one to serve the fresh food compartment at a slightly higher temperature, is a very costly solution to this problem.
A refrigeration system suitable for use in a household refrigerator and having improved thermodynamic efficiency is described in U.S. Pat. No. 4,910,972, which is assigned to the same assignee as the present invention. A system disclosed in U.S. Pat. No. 4,910,972 is shown in FIG. 1. The system comprises a first expansion valve 11, a first evaporator 13, first and second compressors 15 and 17, a condenser 21, a second expansion valve 23, and a second evaporator 25 connected in series in a refrigerant flow relationship by a conduit 26. A phase separator 27 is connected to the outlet of the second evaporator 25 to receive two phase refrigerant therefrom. The phase separator provides liquid refrigerant to the first expansion valve 11 and saturated vapor refrigerant to second compressor 17. The first evaporator is operated at approximately -10.degree. F. and cools the freezer; the second evaporator is operated at approximately 25.degree. F. and cools the fresh food compartment. Thus, this dual evaporator two stage cycle uses much less mechanical energy than the typical single evaporator system.
The above-mentioned related applications, Ser. No. 07/612,051 and 07/612,290, disclose other refrigeration systems having improved thermodynamic efficiency. A system representative of a system disclosed in these applications is shown in FIG. 2. The system of FIG. 2 is similar to that of FIG. 1. However, one difference is that instead of using a multistage compressor unit, the system of FIG. 2 uses a single compressor Particularly, the system comprises a first expansion valve 31, a first evaporator 32, a compressor 33, a condenser 34, a second expansion valve 35, and a second evaporator 36 connected in series in a refrigerant flow relationship by a conduit 37. A phase separator 38 is connected to the outlet of the second evaporator 36 to receive two phase refrigerant therefrom. The phase separator provides liquid refrigerant to the first expansion valve 31 and saturated vapor refrigerant to a refrigerant flow control unit 39. The control unit, which is also connected to the outlet of the first evaporator 32 and the inlet of the compressor 33, selectively allows either refrigerant from the first evaporator 32 or vapor refrigerant from the phase separator 38 to flow to the compressor 33. This system improves efficiency without using multiple compressor stages.
In the multievaporator systems described above, excess refrigerant inventory is normally accumulated in the phase separators. Liquid refrigerant is supplied from the phase separator to the lowest temperature evaporator via an expansion throttle Ideally, the refrigerant will be completely vaporized in the evaporator. However, when the lowest temperature evaporator operates at a temperature which is lower than its design temperature, either due to decreased thermal load or compartment thermostat setting, the refrigerant is not completely vaporized and some refrigerant is discharged from the evaporator as liquid. This liquid refrigerant is effectively stored in the suction line between the lowest temperature evaporator and the compressor unit. Liquid discharge to the suction line represents a loss of cooling capacity because the cooling produced by the evaporation of refrigerant in the suction line is released to the ambient and not the cooled compartment. Also, liquid discharge from the lowest temperature evaporator effectively transfers liquid refrigerant inventory from the phase separator to the suction line Eventually, the phase separator will discharge two-phase refrigerant instead of liquid refrigerant. Consequently, the flow rate through the expansion throttle will decrease.